Prior variable density drilling fluids primarily concerned the use of a highly compressible gas (e.g. air or nitrogen) as a free phase in the fluid. Limited, if any, efforts are made during conventional air, mist or foam drilling to control the expandability of the bulk fluid or to adjust or engineer the compressibility of the fluid other than managing the ratio of air or other gas to the fluid. Other proposals to employ a virtual multiple gradient fluid include so-called dual gradient drilling. This method would use two columns of different density fluids. One column would be essentially static, while the second fluid density is circulated below the seafloor. During drilling the vertical height of the in well bore column would change as the well is deepened and the resulting bulk average fluid density along the wellbore would thus vary with depth.
Typical “single gradient” fluids used today include multiple components (base fluid, various solids and additives). The density of the base fluids is known to vary with temperature and to some degree with pressure. While these density changes are often accounted for during the mathematical modeling of the fluid pressures in the wellbore, the density changes resulting from this behavior is not sufficient to change the design of the wellbore with respect to pore and fracture pressure profiles, as well as position and number of casing strings. No effort is known to be made to intentionally modify the compressibility (density) of classic drilling fluids.
Thus, it is desirable if a true variable density fluid were devised where the properties of the fluid could be designed to fit the requirements of the wellbore operation and the subterranean formations being drilled. It would also be desirable if a variable density fluid composition could be devised that is recirculatable on the current well and/or reused on a second or subsequent well.